Method of operating a ventilation system especially of a motor vehicle

ABSTRACT

PCT No. PCT/EP94/02742 Sec. 371 Date Feb. 13, 1996 Sec. 102(e) Date Feb. 13, 1996 PCT Filed Aug. 18, 1994 PCT Pub. No. WO95/05949 PCT Pub. Date Mar. 2, 1995The previously presented teaching of the invention describes a method for the evaluation of sensor signals with the purpose of producing plausible and physiologically satisfying switching impulses for interrupting or for opening the air supply in ventilation systems, particularly for the ventilation of passenger cabins in motor vehicles, depending on the pollutant content of the ambient air established by one or more sensors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of PCT/EP94/02742 filedAug. 18, 1994 and based, in turn on German application P 43 28 218.0 ofAug. 21, 1993 under the International Convention.

FIELD OF THE INVENTION

The present invention relates to the use of sensors for the purpose oftriggering an electric impulse when there are increased concentrationsof gases (target gases) to which the sensor element or elementssensitive. (=target gases).

More particularly, an application of the invention is interruption ofthe air supply to a motor vehicle cabin when due to the emissions ofother vehicles, there is an increased concentration of noxious or toxicgases such as carbon monoxide, nitrogen oxides, sulfur dioxide,partially burnt hydrocarbons, etc. in the air supply to the vehiclecabin.

BACKGROUND OF THE INVENTION

It is known to use various sensors for detecting concentrations ofvarious gases, for instance:

    ______________________________________                                        Tin dioxide (SnO.sub.2)                                                                           for all oxidizable gases                                  Phthalocanine       for reducible gases                                       Tungsten trioxide (WO.sub.3)                                                                      for nitrogen oxides.                                      ______________________________________                                    

For the evaluation of the signals it is known to trigger the switchingsignal not when an absolute level is reached, but rather then when thesensor or sensors detect an increased concentration of the target gas inrelation to the average level.

Since the sensors are subject to a certain drift, being mostlycross-sensitive with respect to air humidity, and have to operate inareas with very different basic pollutant levels, it has been proposedto differentiate the sensor signal from the background via an electrichigh pass filter so that the noxious substances resulting from othermotor vehicles give rise to clearly differentiated switching pulses tocontrasted with the above-mentioned background effects. If the basicfrequency of the high-pass filter is selected so that in contrast to theslower changes these pulses (target impulses) are transmitted, eachrelevant deterioration of the level of noxious substances willadvantageously lead to a switching signal. It has been further proposedto differentiate the signal many times, so that even smaller signalchanges can be evaluated (see EP 87 00 592).

Since, as a rule, sensors react quickly to a target gas, but aresignificantly slower in restoring to the initial value, this method isonly satisfactory for generating the command "interrupt air supply", butthe cancellation command "reopen air supply" is not produced assatisfactorily.

It is known to continuously build up the average value of the sensorsignal over a certain time period (e.g. for 10 minutes) and to comparethe actual (instantaneous) sensor value with this average value. Thesignal "close air supply", as well as the signal "open air supply" canbe derived therefrom satisfactorily. Details are described in EP 87 00592 from 1986/87. However for this method sensor signals of a certainamplitude are required. In the case of very small sensor signals,uncertainties in their evaluation occur.

SUMMARY OF THE INVENTION

According to the invention it is therefore proposed to factor in thesignal changes as such for evaluation, whereby the consideration ofbehavior over time plays an essential part.

In particular, the method for evaluating sensor signals has the purposeof generating switching pulses which, via suitable setting elements andair supply ducts, influence the air supply to an inner space, preferablythe cabin of a motor vehicle. The air supply is interrupted in thepresence of a gas concentration representing a switching criterium orthe supplied air is passed through a filter system before it reaches theinner space, i.e. the supply of air from a particular source is cutoff.The initial state is restored, i.e. air supply from the original sourceis restored, when the quality of the ambient air improves.

When the sensors have an electrical resistance which reduces in thepresence of the target gas, an integral is formed from the actual sensorvalue over a free determined time period which is reduced by a value (x)with the free air supply being interrupted as soon as the actual sensorvalue falls below the threshold formed by the integral reduced by thevalue (x). The switching signal can also be triggered when theelectronic evaluation system determines that the change rate of thesensor signal in a free determinable time period is greater than a freedetermined value (y).

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a graph of resistance versus time illustrating thecharacteristic curve of a sensor responding to the development of atarget gas concentration in the ambient air to be supplied to a closespace;

FIG. 2 is a block diagram of a control system according to theinvention; and

FIGS. 3 through 8 are graphs of resistance versus time showing variousembodiments of control according to the invention.

SPECIFIC DESCRIPTION

FIG. 1 shows a change in a typical sensor signal provoked by targetgases. At 1.1 the sensor is less subject to pollutants (normal airvalue). The content of target gases in the air starts at the point oftime 1.2. Here a negative gradient can be reliably detected and has aslope which is clearly greater than the normal variations. During thepresence of target gases the sensor is in the area 1.3. When thepollutant content of the air returns to normal, an increase in thesensor value can be at first noticed at 1.3, whereby after a while theinitial value 1.4 is reached.

It is thus advisible to interrupt the air supply at the point in time1.2 and to restore it again at the point in time 1.3. In the example asensor is used whose electric resistance drops under by the action ofthe target gases.

In principle the above applies also when the sensor has the reversecharacteristic, which means that its resistance increase in the presenceof the target gas.

However in practice the sensor signals are rarely so clear cut, so thatthe signal processing has to meet higher demands.

In the preferred embodiment (FIG. 2) the evaluation of the sensorsignals 2.4 takes place in a program-controlled microcomputer 2.3, withthe signals from the sensor 2.1 being digitized in an analog/digitalconverter 2.2.

FIG. 3 shows the principle of the invention in a very simplified manner.The sensor signal is shown at 3.1 and by nature has comparatively smallvariations. The curve 3.2 is a mathematical average value (integral) ofthe signal 3.1, whereby the integration time can be freely determinedand as a rule is 5-10 min. The curve 3.3 is a computer-created curvewhich has numerical values which are lower by a small amount (=x) thanthe curve 3.2. The amount x can be a fixed amount or it can be aproportional factor or e.g. 1% of the curve 3.2.

The air supply is interrupted at a point in time 3.4, when the curve 3.1assumes a lower value than the curve 3.3 and is again opened when thevalue 3.1 is again greater than the value 3.3. It can be thereby noticedthat the integration time has a considerable influence.

If the integration time is relatively short, the ventilation would berestored even though there was an objectively high level of noxioussubstances. On the contrary, if the integration time is very long, insome situations the closing time will also be subjectively too long.

Very high concentrations of target gases generate in the sensor, at themoment of detection, a very quick signal change. For this situation theinvention proposes advantageously to always trigger an additionalswitching signal when the differential rate of the sensor signal overtime is greater than a certain predetermined value (y), independently ofthe fact whether the switching condition previously described in FIG. 3is met. FIG. 4 describes the principle:

The sensor value 4.1 undergoes a quick change when subjected to theaction of the target gas. Before it reaches the threshold 4.2 thefollowing is established by the evaluation computer 2.3:

    Δs/Δt>y

In this way a further switching condition is met and the switchingimpulse is triggered.

In the explanation of FIG. 3 it was noted that the cancellation of theclosing command, i.e. the command "admit air supply" functions with thedescribed method, but in certain situations remains physiologically andobjectively unsatisfactory.

FIG. 5 shows an advantageous solution. The curve 5.1 is the curverepresenting the actual sensor signal and the curve 5.2 is the integralof the sensor signal minus x. At the point of time 5.3 the air access isinterrupted. At the point in time 5.4 the sensor value has achieved apositive gradient. The criterium for opening the air supply is met whenthe actual sensor value, starting out from the point in time 5.4,reaches a value 5.5. increased by the value y. Thus the integration timeis no longer the only defining factor of the switching behavior. Theinvention uses to its advantage the observation that the human olfactoryorgan can adapt to "bad air", and basically reacts only to changes inthe air composition, or the air pollutant content.

If the arrangement according to the invention reacts to a so-called"fresh air pulse", independently of the objective air quality, this willbe acceptable as to the human nose.

FIG. 6 shows a system of this latter type. In pollutant-free air thesensor assumes the value 6.1. In the presence of the target gas thevalue 6.3 is reached. The initial value (fresh air level) is shown inbroken lines as 6.2. Now the sensor is subjected to a further pulse ofnoxious substances and produces the reaction 6.5. The reaction, whichseems insignificant at a first glance, in fact represents the result ofan pulse of noxious substances, similar to 6.3. However this pulse 6.5hit on a sensor in the desorption phase 6.6. If the starting point wasthe fresh air level 6.2, than a signal similar to 6.4 would have beenregistered, whose greater gradient would have met the switchingcriterium described in FIG. 4.

Therefore in accordance with the invention the computer 2.3 establishesat all times the value and direction of the respective slope. In thedesorption phase 6.6 a positive change rate of D1 is found. At themoment of gas impulse it comes to direction reversal and the nownegative change rate equals D2 at 6.5. The computer program now adds theabsolute values of the two change rates

    abs (D1)+abs (D2)=D3

D3 is the change rate to be evaluated. The program checks now whetherthe change rate is greater than a value y, which would lead to turningoff the air supply without delay. Depending on the characteristics ofthe used sensor element, and particularly on its time lag in thedesorption phase 6.6., it is proposed to multiply the change rate D1 bya factor, in order to obtain with D3 a value which corresponds to thechange rate 6.4 in the case of the nondesorbing sensor.

With the previously described teaching a satisfactory switching behavioris achieved in most cases. Especially then when pulses of the target gasare directed in quick succession onto the sensor or sensors, theresulting situations are not always satisfactory and plausible. For thisreason the method of control is effected according to FIG. 7 as followedis proposed:

The initial sensor value is 7.1, which diminishes in the presence of thetarget gas and at the point of time 7.3 falls below the value 7.2resulted from the integral, thereby fulfilling the switching criterium.Without target gas the sensor recovers again and shows a positivegradient, which at the point in time 7.7. has reached the value y, whichtriggers the switching impulse (open air supply). At the point of time7.4 a reversal of the gradient can be seen, which is caused by a newoccurrence of the presence of target gas. At this moment the comparativevalue 7.2 is set to the value 7.6, which is the actual sensor value atthe point of time 7.4 lowered by the rate x known from FIG. 3.

At the point of time 7.8 the actual sensor rate runs through the curve7.6 and thereby triggers again the switching signal "interrupt airsupply".

In the method of the invention illustrated in FIG. 8, the formation ofthe comparative value 8.2 and 8.4 is made dependent on the rate anddirection of the gradient assumed by the actual sensor value 8.1 and8.5. The reason for that is the observation that certain sensors have aclearly longer desorption phase compared to the adsorption phase.

In the desorption phase the integration takes place mostly in the sensoritself, so that a further integration makes the resetting of the of thecomparative value too slow. According to the invention, we create thecomparative value 8.4 after the direction reversal of the gradient 8.3either directly from the actual sensor value, diminished by a value x,or from an integral with reduced integration time. If the rate of thegradient is smaller than a freely selected value, then again theintegration of the sensor values takes place over a longer time span.

The teaching of the invention is advantageously applied in a versionaccording to FIG. 2, whereby the execution is assigned to aprogram-controlled microprocessor as a purely mathematical operation.

We claim:
 1. A method of operating a ventilation system, comprising thesteps of:(a) supplying air to a space to be ventilated and formed by acabin of an automotive vehicle; (b) detecting a concentration of atarget gas in air to be supplied to said space and generating an actualvalue electrical signal as a measure of said concentration; (c)automatically forming an integral value by integrating said actual valuesignal over a selected time period; (d) automatically generating aswitching signal and cutting off supply of said air to said space withsaid switching signal selectively upon:(d₁) said actual value signalvarying through a first threshold determined by said integral value andin a direction representing a certain increased concentration of saidtarget gas, and (d₂) a rate of change of said actual value signal insaid direction over a preselected time interval exceeds a preselectedvalue; (e) automatically canceling said switching signal and restoringsupply of said air to said space upon a change in said actual valuesignal in an opposite direction; and (f) monitoring a gradient in saidactual value signal after cancellation of said switching signal andrestoration of supply of air to said space and, upon reversal of saidgradient, adding an absolute value of the negative gradient to anabsolute value of the positive gradient, and triggering a switchingpulse to again cut off flow of air to said space when the sum ofabsolute values of said gradients is greater or said actual value signalis less than said further value.
 2. The method defined in claim 1wherein the concentration of said target gas is detected with a sensorwhich has reduced electrical resistance as the concentration of saidtarget gas increases and said switching signal is automaticallygenerated when said actual value signal falls below the integral valuediminished by a predetermined value (x) and the switching signal iscancelled and supply of air is restored to said space upon a rise ofsaid actual value signal said actual value signal being a signalrepresenting said resistance.
 3. The method defined in claim 1 whereinsaid concentration of said target gas is detected in air supplied tosaid space with a sensor which increases electrical resistance in thepresence of the target gas and said switching signal is generated whensaid actual value signal rises to said integral value and said rate ofchange is a positive gradient.
 4. The method defined in claim 1 whereinsaid switching signal is cancelled and supply of said air to said spaceis restored upon a rise in said actual value signal to a thresholdrepresenting said integral value reduced by an amount (x).
 5. The methoddefined in claim 1 wherein said switching signal is cancelled and supplyof said air to said space is restored when a signal from a sensormeasuring said concentration is reversed by desorption at said sensor,an actual sensor signal is stored in an electronic evaluation system andis increased by an amount to a switching level and the supply of air tosaid space is restored when the actual value signal rises above a secondthreshold corresponding to said switching level.
 6. The method definedin claim 1 wherein a sensor is added for the measurement ofconcentration of said target gas which reverses in signal output upondesorption, an actual sensor signal at desorption is stored and thestored signal is increased by a certain amount of to a switching leveland said switching signal is automatically cancelled and supply of airto said space is restored upon the actual value signal falls below athreshold determined by said switching level for a sensor which hasincreased resistance in the presence of said target gas.
 7. The methoddefined in claim 1 wherein formation of said integral value isinterrupted when the gradient of said actual value signal becomesnegative in a case in which said sensor is a sensor measuring saidconcentration and having a resistance increasing with the presence ofsaid target gas and, when said gradient is negative, said threshold isformed from the actual sensor value plus a value (x).
 8. The methoddefined in claim 1 wherein said threshold is determined by deducting avalue (x) from the integral value and, upon a reversal of a gradient ofsaid actual value signal, the integration duration is shortened with alonger normal integration time being restored when said gradient isrepresented by a quotient smaller than a given value.
 9. A method ofoperating a ventilation system, comprising the steps of:(a) supplyingair to a space to be ventilated and formed by a cabin of an automotivevehicle; (b) detecting a concentration of a target gas in air to besupplied to said space and generating an actual value electrical signalas a measure of said concentration; (c) automatically forming anintegral value by integrating said actual value signal over a selectedtime period; (d) automatically generating a switching signal and cuttingoff supply of said air to said space with said switching signalselectively upon:(d₁) said actual value signal varying through a firstthreshold determined by said integral value and in a directionrepresenting a certain increased concentration of said target gas, and(d₂) a rate of chance of said actual value signal in said direction overa preselected time interval exceeds a preselected value; (e)automatically canceling said switching signal and restoring supply ofsaid air to said space upon a change in said actual value signal in anopposite direction; and (f) monitoring a gradient in said actual valuesignal after cancellation of said switching signal and restoration ofsupply of air to said space and, upon reversal of said gradient, addingabsolute value of the negative gradient to an absolute value of thepositive gradient and triggering a switching pulse to again cut off flowof air to said space when the sum of absolute values of said gradientsis smaller than a further valve or said actual value signal is greaterthan said further value.
 10. A method of operating a ventilation system,comprising the steps of:(a) supplying air to a space to be ventilatedand formed by a cabin of an automotive vehicle; (b) detecting aconcentration of a target gas in air to be supplied to said space andgenerating an actual value electrical signal as a measure of saidconcentration; (c) automatically forming an integral value byintegrating said actual value signal over a selected time period; (d)automatically generating a switching signal and cutting off supplv ofsaid air to said space with said switching signal selectively upon:(d₁)said actual value signal varying through a first threshold determined bysaid integral value and in a direction representing a certain increasedconcentration of said target gas, and (d₂) a rate of change of saidactual value signal in said direction over a preselected time intervalexceeds a preselected value; (e) automatically canceling said switchingsignal and restoring supply of said air to said space upon a change insaid actual value signal in an opposite direction; and (f) interruptingsaid integral value when a gradient of the actual value signal becomespositive in a case in which said concentration is measured with a sensorhaving a reduced resistance in the presence of said target gas and, whensaid gradient is positive, forming a threshold by deducting an amount(x) from the actual sensor value.